In general, an optical fiber is composed of a core region and a cladding region. The core region is made of a silica glass of which a refractive index is increased by doping, for example, a germanium. The cladding region is made of a silica glass having a refractive index lower than that of the core region, which forms a layer surrounding a circumference of the core. A light propagates through the core region by a total reflection of the light at a boundary between the core region and the cladding region. Conventionally, a relative refractive index difference between the core region and the cladding region is at largest 3% to 4%.
On the other hand, in recent years, an optical fiber has been reported with which a large relative refractive index difference can be obtained compared to the above structured optical fiber (see, for example, Patent Literature 1). According to the Patent Literature 1, it has been reported that an average refractive index of the cladding region can be greatly reduced by placing a microstructure, which is formed by arranging a plurality of holes in the glass of the cladding region, in a longitudinal direction. In other words, the optical fiber having the above microstructure can dramatically increase an effective refractive index of the core region compared to the conventional optical fiber.
Given this situation, in recent years, a microstructured optical fiber has been a focus of constant attention, in which a hole or the like is formed in a surrounding area of the core region of an optical fiber having a refractive index profile structure equivalent to a typical single mode optical fiber (SMF). For instance, it has been reported that a macro-bending loss against a small-diameter bending with a diameter of 15 mm (macro-bending loss r=15 mm) could be lowered to 0.04 dB/m by placing the microstructure (see, for example, Nonpatent Literature 1). After that, there has been another report that the macro-bending loss could be further lowered up to below 0.01 dB/m (see, for example, Nonpatent Literature 2). On the other hand, an optical fiber has been proposed, in which the macro-bending loss is lowered while maintaining a large mode field diameter (MFD) in view of connection with the conventional SMF (see, for example, Patent Literature 2).
The optical fiber described in the Patent Literature 2 includes a microstructure in which a hexagonal lattice array of sub-medium region is formed in multilayer in the cladding region placed on a circumference of the core region. In other words, the optical fiber has a microstructure in which the sub-medium region is formed in multilayer at six folds rotationally symmetric centering on the core region. The sub-medium region is composed of an air, a liquid, or a glass filling each of the holes formed in the hexagonal lattice array in multilayer in the cladding region, having a refractive index lower than that of a main medium of the cladding region.
The microstructure in which the sub-medium region is formed in multilayer at the six folds rotational symmetry (hereinafter, “microstructure of the six folds rotational symmetry”) is effective as a structure for an optical fiber that lowers the macro-bending loss, propagating a signal light of a predetermined wavelength band in a single mode.    Patent Literature 1: Japanese Patent No. 3306847    Nonpatent Literature 1: T. Hasegawa, et al., Microoptics Conference (2003), K2    Nonpatent Literature 2: Daizo Nishioka, et al., Technical Report of IEICE, OFT 2003-63, P. 23    Patent Literature 2: Japanese Patent Application Laid-Open No. 2004 220026